مروری بر مطالعات آندروژن و استروژن در مراحل اولیه زندگی ماهی: اثرات بر روی ژن و کنترل هورمونی تمایز جنسی A review of studies on androgen and estrogen exposure in fish early life stages: effects on gene and hormonal control of sexual differentiation

INTRODUCTION There is growing concern about hormones in the environment. This stems from the fact that both natural and synthetic hormones have been detected in aquatic environments. Main sources of hormones are runoff from lands in which animal manure has been applied as a fertilizer and discharge of treated municipal wastewater into bodies of water. Even at low concentrations (parts per trillion to low parts per billion) synthetic estrogens and androgens can have high potency to teleost fishes, especially in sensitive early life stages. Fish have been used as model organisms to study the effects of androgen and estrogen exposure; however, much of this research has been conducted in adult fish. There is much less literature available on the effects of these hormones on fish at early life stages, especially during periods of enhanced sensitivity such as sex determination and differentiation. The purpose of this review is to determine what is currently known about the effects of environmental hormones on early life stage fishes, including potential sources of hormones and modes of hormone action. We begin the review with an overview of the state of hormones in the environment and their potential sources, including human and animal sources. We continue with an overview of the current state of knowledge of sex determination and sex differentiation in teleost gonachorist (unisexual) fishes, as these processes can be greatly influenced by environmental hormones. We then present a summary of studies that have examined the effects of androgens and estrogens on fish sex determination and differentiation after exposure of fish embryos and larvae, and conclude with ideas for future research. NATURAL AND SYNTHETIC HORMONES IN THE ENVIRONMENT Human Sources A summary with information regarding steroidal hormones reviewed in this article is presented in Tables 1 and 2. One of the major sources of hormones to the environment is effluent from sewage wastewater treatment plants (SWWTP). Women excrete a range of natural estrogens, with pregnant, premenopausal and postmenopausal women excreting 3115, 6.7 and 3.6 µg 17β‐estradiol (17β‐E2) equivalence per day, respectively (Liu et al., 2009). Men excrete testosterone (T), dihydrotestosterone, androstenedione and androsterone at rates of 81 µg T equivalence  per day (Liu et al., 2009). Although studies have shown that aqueous phase removal efficiencies in treatment plants can be as high as 99%, effluents may still contain environmentally unsafe hormone concentrations (Chimchirian et al., 2007). The results obtained by Chimchirian et al. (2007) suggest that the treatment processes that employ suspended growth media (e.g. activated sludge) are more effective at removing estrogens from influent than static biofilms (e.g. packed beds). Liu et al. (2009) conducted a review of estrogens and androgens detected in SWWTP influent and effluent in several countries. Their review found that the USA had the greatest effluent concentrations of androstenedione (up to 7720 ng l−۱ ), T concentrations similar to those in Canada (up to 20 ng l−۱ ) and estrogen concentrations as high as 49, 20 and 140 ng l−۱ for estrone (E1), 17β‐E2 and estriol (E3), respectively. Healthcare facilities can contribute significant hormone loads to SWWTP due to patient excretion and pharmaceutical disposal. Nagarnaik et al. (2010) found that a hospital generated hormone mass loadings of 92 mg per day, with androgens constituting more than 65% of the total load. The potential estrogenicity of the hospital wastewater effluent was found to be 130 ng l−۱ as 17β‐E2. Chimchirian et al. (2007) reported significantly higher 17β‐E2 concentrations in influent to a SWWTP receiving hospital wastewater compared with two others only receiving municipal wastewater; however, the samples were not analyzed for androgens. In addition to the contribution of hormones to the environment from treated sewage, hormones also are released to the environment from untreated sewage by combined sewer overflow (CSO) events and the application of biosolids to agricultural fields. Pailler et al. (2009) investigated the contribution of CSO events to hormone loads in a stream in Germany by sampling downstream of nine CSO locations and a SWWTP. They collected stream samples over hydrographs during 11 flood events and calculated 17α‐ethinylestradiol (EE2), 17β‐E2 and E1 loads of up to 122, 78, and 274 mg per storm event, respectively. The average estrogen concentrations observed in the stream during these events were ca 5–۶ ng l−۱ each for EE2, 17β‐E2 and E1. Langdon et al. (2010) reviewed the literature to identify ranges of concentrations of chemicals in biosolids. The estrogens detected in biosolids were EE2, 17β‐E2 and E1 with ranges of 0.42–۱۷, ۰٫۳۱–۴۹ and nondetected–۱۵۰ µg kg−۱ , and means of 4.01, 13.5 and 10.9 µg kg−۱ , respectively. In 1998, the US Environmental Protection Agency (USEPA) estimated that 2.8 million dry tons of biosolids were land applied, and estimated that, by 2010, as much as 8.2 million dry tons could be applied (USEPA, 1999). Therefore, in 2010, an average of 0.23 tons of estrogens were potentially introduced into the environment from biosolids applications. The predicted maximum runoff concentrations from soils amended with biosolids were estimated to be 0.01, 0.03 and 0.24 µg l−۱ , respectively, with E1 expected to be the most mobile and EE2 the least mobile.